Contamination of subsurface soils is a matter of great concern and can cause many environmental problems if left in the subsurface soil. This contamination of subsurface soil can occur in a number of ways, such as accidental spills, leaks from storage facilities or tanks (in the past, it was not uncommon for gasoline storage tanks under service stations to leak and contaminate the soil beneath the storage tanks), landfill seepage, etc. Sometimes the contamination is a result of an industrial process or resource extraction.
In most circumstances it is desirable or even necessary to deal with the subsurface soil contamination. Sometimes clean-up of the contamination is even required by regulation.
The most basic method of treating contaminated subsurface soil is also the most labor intensive; simply excavating the contaminated soil and hauling it off-site for disposal, such as by incineration. The problem with this method is that although it is straightforward and rather uncomplicated, it is very labor and resource intensive. Because this method is so labor intensive and uneconomical in all but the rarest circumstances, there have been numerous attempts at developing in-situ soil remediation, where the contamination is addressed in the subsurface soil without having to make extensive excavations. Of these in-situ soil remediation methods, various methods have been tried or used such as injection techniques, where chemicals or biological agents are injected in-situ in the soil to treat the contaminant within the subsurface soil.
One of the more popular in-situ soil remediation methods that is commonly used is soil vapor extraction or SVE. SVE applies a vacuum to a well hole drilled vertically in to the contaminated subsurface soils. When the vacuum is applied, air is forced to travel through the pore spaces in the soil, causing liquid in the soil to volatize and be carried out. The air carries the volatized contaminants and the volatized contaminants are removed from the well hole by the vacuum created. The extracted gases are then either vented to atmosphere or into a containment system for later treatment.
However SVE works best in highly permeable soils because it allows a greater area around the well bore to be treated by the SVE. The more permeable the soil, the greater the area of surrounding soil that will allow volatized contaminants to be drawn out by the vacuum created in the well hole. The less permeable the soil, the smaller the area of subsurface soil around the well hole that will be treated by the SVE and the effectiveness of SVE methods will quickly diminish in less permeable soils.
To increase the effectiveness of soil vapor extraction methods, a number of different methods have been tried and used to try to increase the effective range of these soil vapor extraction methods surrounding the down hole. Because of this requirement that the subsurface soil have relatively high permeability for SVE to be really effective, a number of methods and treatments have been used to try and increase the permeability of the soil surrounding the well hole. Some of these methods include: pneumatic fracturing where air is forced down the well hole in pressure waves in an attempt to create fractures in the surrounding soil to increase the permeability of the soil; and heat treating methods to decrease the viscosity and increase the volatility of fluids in the soil.
In an effort to increase the effects of soil vapor extraction electric soil heating systems have been tried. Soil heating imparts an electrical field across a contamination zone to cause contaminants to move towards the electrodes. However, previous attempts have not had very consistent or promising results.